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PORTLAND, Ore. — Want to know when that baby elephant will be arriving? Join the herd.

As Asian elephant Rose-Tu heads into her 21st month of pregnancy, anticipation is building at the Oregon Zoo. Some staffers have organized informal office pools to guess the date of the calf’s arrival.

Despite years of important scientific discoveries about elephants — many of them made right here in Portland — pinpointing a pregnant elephant’s due date remains something of a mysterious art.

With a 22-month gestation period, the longest of any mammal on the planet, giving birth “a little early” can mean several weeks. Currently, the best predictor is blood-progesterone level, which remains high throughout pregnancy and drops precipitously a few days before delivery.

“Elephant pregnancy is long and the end point is difficult to predict more than a fewdays in advance,” said zoo research associate Heather Velonis, a graduate student in biology at Portland State University. “But if we can identify specific, predictable behaviors that indicate when an elephant will give birth, we might be able to have two weeks’ notice instead of two days.”

Velonis, with the help of zoo conservation research associate Karen Lewis and zoo volunteers, is looking to turn the guessing game into a science. By carefully observing Rose-Tu in the weeks leading up to birth, researchers are seeking behavioral clues that might enable them to accurately predict the date ahead of time.

Velonis and a team of trained observers are collecting data for 30 minutes at a time, four times a day, seven days a week, based on an ethogram — a checklist of specific behaviors. For example, Rose-Tu might stay away from the other elephants in the herd, swing her tail more often, or drink less water than usual. She might throw dust at her belly, lie down, place her trunk in her mouth more frequently, or freeze in an awkward position, then release. Elephant keepers have observed all of these behaviors through the years, but until now they’ve just been part of the anecdotal record.

Lewis initiated the current study in 2008 when Rose-Tu was pregnant with Samudra. Researchers are also collaborating with the Taronga Zoo in Sydney, Australia, where three elephants have given birth since 2008. Velonis plans to write up the results of these five births for her master’s thesis and for publication in a scientific journal.

The Oregon Zoo’s string of successful elephant births — more than 25 since the early 1960s — has played a key role in establishing what is already known about elephant gestation. The zoo’s most famous elephant, Packy, was born in 1962 — the first successful elephant birth in the Western Hemisphere in nearly 44 years. At the time, the pregnancy of Packy’s mother, Belle, was a huge milestone for zoos.

“We’ve come a long way,” Lewis said. “When Packy was born, nobody knew how long an elephant pregnancy lasted or what signs to look for. The zoo’s veterinarian at the time, Matthew Maberry, spent three months sleeping in the elephant house.”

By observing Belle, Dr. Maberry and the zoo’s elephant keepers were able to calculate the length of gestation, which to that point had been an unknown. After Packy’s birth, 14 more elephants were born at the Oregon Zoo over the next dozen years — no other zoos had elephant births during this period — and “Doc Maberry,” as he was popularly known, developed an excellent eye for predicting due dates. The zoo learned a lot about new calf development and maternal care and shared this important information with other zoos, which began to see their own elephant calves from 1975 onward.

The Oregon Zoo is recognized worldwide for its successful breeding program for Asian elephants, which has now spanned 50 years. Rose-Tu’s mother, Me-Tu, was the second elephant born at the zoo (just months after Packy in 1962), and her grandmother, Rosy, was the first elephant ever to live in Oregon.

Asian elephants are considered highly endangered in their range countries, threatened by habitat loss and conflict with humans. It is estimated that only 40,000 to 50,000 remain in fragmented populations from India to Borneo.

The Oregon Zoo is an accredited member of the Association of Zoos and Aquariums, whose species survival plan for Asian elephants is striving to establish a self-sustaining population in North America.

The zoo is a service of Metro and is dedicated to its mission of inspiring the community to create a better future for wildlife. Committed to conservation, the zoo is currentlyworking to save endangered California condors, Oregon silverspot and Taylor’s checkerspot butterflies, western pond turtles and Oregon spotted frogs. Other projects include studies on Asian elephants, polar bears, orangutans and giant pandas. The zoo relies in part on community support through donations to the Oregon Zoo Foundation to undertake these and many other animal welfare, education and sustainability programs.

The zoo opens at 9 a.m. daily and is located five minutes from downtown Portland, just off Highway 26. The zoo is also accessible by MAX light rail line. Visitors who travel to the zoo via MAX receive $1.50 off zoo admission. Call TriMet Customer Service, 503-238-RIDE (7433), or visit www.trimet.org for fare and route information.

General zoo admission is $10.50 (ages 12-64), $9 for seniors (65 and up), $7.50 for children (ages 3-11) and free for those 2 and younger; 25 cents of the admission price helps fund regional conservation projects through the zoo’s Future for Wildlife program. A parking fee of $4 per car is also required. Additional information is available at www.oregonzoo.org or by calling 503-226-1561.